Various tap configurations are known in the art, produced by company Rapak (UK) (shown for example in FIGS. 18 and 19, which are known embodiments of this product).
In any case, there are also other similar configurations of a known type, related to taps actuated by connectors having similar principles to those of the above mentioned tap, for example marketed by companies ITW New Zealand Limited (US-A1-2004238778) and Rieke Corp (U.S. Pat. No. 6,378,742).
All known taps listed above have approximately a similar operating/actuating principle, namely, with reference to FIG. 18:                step 1: screwing-type connectors 7 present on the market;        step 2: screwing the connector 7 onto a body 14.3 of the tap 14;        step 3: internal thrust of the connector with an OR-ring 13, which will perform an operating sealing onto the internal walls of the upper part of the tap (an opposite threading) 14;        step 4: the bit 8.1 of an internal peg 8 to the connector 7 will centrally push a central actuator 14.1 of the tap, which will free the outlet hole of liquid from the tap present on the body 14.3; the central actuator 14.1 will move by a preset height till it blocks itself on a plane element or surface of the body 14.3, and at that time the central actuator 14.1 will operate as fixed abutment to open the centre of the internal peg 8 and free the connector passage, creating an open channel between connector and tap;        step 5: behind the central actuator 14.1 of the tap a metallic spring 14.2 is assembled (or the actuator itself will have the shape of an elastic dome, supported also by a metallic spring therein) which allows its automatic closure (following the reverse cycle to the one described above), allowing to take back the two systems, connector and tap, to their closing position.        
Such taps however have some defects. For example, some of them are not completely made of plastic material, but make use of metallic springs therein (as occurs for the Rapak (UK) tap) to enable the tap, once disconnected from the connector, to go back to its closing position avoiding liquid leakages.
The presence of a metallic spring in a set of plastic pieces makes the tap not eco-compatible in terms of recycling, unless one decides to disassemble every single tap, differentiating plastic from metal.
The element of the main body is further complex to be able to create the stable seat of the metallic spring, or anyway of the spring elements present on the plug itself.
This highly increases the manufacturing and building costs of the die, in addition to those for assembling the piece.
The flow of such taps is often limited by the fact that the opening elements of the tap 14 itself and of the connector 7, and namely of the central actuator 14.1 and the internal peg 8, being both in a central position, will “clog” the passage of liquids, strongly increasing the flow from the system.